1. Field of the Invention
The present invention relates to an image forming apparatus that reverses the front and back of recording paper by performing switchback transport of the recording paper to perform duplex printing of the recording paper.
2. Description of the Related Art
Ordinarily, in an image forming apparatus such as a copy machine, a printer, or a facsimile device, an electrostatic latent image is formed on a photosensitive body, toner is supplied to the photosensitive body from a development apparatus, the electrostatic latent image is made visible on the photosensitive body by the toner, a toner image is formed on the photosensitive body, the toner image is transferred from the photosensitive body to recording paper, heat and pressure are applied to the recording paper, and thus the toner image is fixed on the recording paper.
In a fixing apparatus, heat and pressure are applied to recording paper sandwiched between a hot roller and a pressure roller (also referred to as a fixing roller), so that the toner image is hot-melted on the recording paper and thus fixed.
Incidentally, when a toner image is transferred to an entire sheet of recording paper, and the recording paper is passed between the rollers of the fixing apparatus, it is difficult for the ends of the recording paper to peel away from the rollers due to the adherence of toner to the ends of the recording paper, so the recording paper curls around the rollers and jams may occur.
Therefore, often a void area where a toner image is not transferred is formed at the ends of the recording paper. In the void area at the ends of the recording paper, a toner image is not transferred and so toner adherence does not occur. Thus, even after heat and pressure have been applied to the recording paper between the rollers of the fixing apparatus, the ends of the recording paper can easily be peeled away from the rollers, so the recording paper does not curl around the rollers and jams do not occur.
On the other hand, even when a toner image is transferred to the trailing end of the recording paper, the trailing end of the recording paper is quickly separated from the rollers after the trailing end of the recording paper has passed between the rollers, so a void area is not formed at the trailing end of the recording paper. Conversely, attempts have been made to reduce the margin area of the trailing end of the recording paper as much as possible (see JP H10-86350A).
With respect to printing of recording paper, a margin portion may be provided at the leading end, trailing end, and both sides of the recording paper. These margin portions are set as one item of the print layout of the recording paper, and differ from the void area of the leading end of the recording paper for preventing curling around the rollers. For example, even if the margin portion of the leading end of the recording paper for the print layout has been set to none, a void area of the leading end of the recording paper separate from the margin portion will remain.
Recently, there have been increasing demands for a recording paper duplex printing function in image forming apparatuses. In this duplex printing function, for example, after transfer and fixing of a toner image to the front face of the recording paper ends, transport of the recording paper is temporarily stopped, the recording paper is transported in the opposite direction to the previous stage of the photosensitive body via a detour path, the front and back of the recording paper are reversed, and transfer and fixing of a toner image to the back face of the recording paper are performed.
This sort of transport in order to reverse the front and back of the recording paper is referred to as switchback transport, and when reversing the front and back of the recording paper with this switchback transport, the leading end and the trailing end of the recording paper are switched, so the back face of the recording paper is printed from the trailing end of the recording paper.
However, as previously stated, when printing to the front face of the recording paper, a void area is not formed at the trailing end of the recording paper, so a toner image is printed up to the trailing end of the recording paper. When afterward printing to the back face of the recording paper, when the recording paper is passed between the rollers of the fixing apparatus beginning with the trailing end of the recording paper, it is difficult for the trailing end of the recording paper to peel away from the rollers due to the adherence of toner to the trailing end of the recording paper, so the recording paper curls around the rollers, and so the problem arises that jams occur.